The subject matter described in the present disclosure was developed with U.S. Low frequency acoustic and seismo-acoustic projectors find applications in marine seismic survey, underwater, ocean acoustic tomography, long-range acoustic navigation and communications. All these applications need powerful and efficient sound sources in a low frequency range such as frequency range 5-100 Hz.
The low frequency source can be one of various impulse types such as explosive (e.g. dynamite), air-guns, plasma (sparker) sources and boomers, or marine vibrators (vibroseis) providing continuous frequency sweeps. Seismic air-gun surveys such as those used in the exploration of oil and gas deposits underneath the ocean floor, produce loud, sharp impulses that propagate over large areas and increase noise levels substantially. While air-guns conventionally are traditional tools to image formations, structures, and deposits deep in the ocean substrate, they also have drawbacks from an engineering/industry point of view. They produce high power non-coherent sound pulses and often radiate in directions other than those required for hydrocarbon exploration. Also, their signal is not highly controllable, either in frequencies content or stability. Marine Vibrators are a coherent type of sound source, which can be a quieter and less harmful technology. In addition, such a sound source provides clearer, more precise and higher resolution imaging of the bottom properties due to their coherent signal and streamer array processing. Reducing and managing the impact of oil exploration on marine mammals may be important to oil and gas producers. Application of quiet and highly coherent marine vibrators as a replacement for the traditional air-gun technology has been an increasing focus of oil and gas producers.
Current continuous wave type sources make use of hydraulic, pneumatic, piezoelectric or magnetostrictive drivers and different types of resonance systems to store acoustic energy and to improve impedance matching, when generating low-frequency sound waves in water. The power output of a simple acoustic source is proportional to the squares of volume, velocity, and frequency and needs a large vibrating area to achieve reasonable levels. As a result, the sound source can become unacceptably large and expensive.
Seismic sources in the form of an underwater gas-filled balloon (or bubble) have been proposed and patented, for example in U.S. Pat. Nos. 10,139,503, 9,383,463, 8,634,276, 8,441,892, 8,331,198, the entire disclosures of which are hereby incorporated by reference herein. A resonant bubble seismic source is a simple, efficient, narrow-band projector. Seismic survey applications may demand a large frequency band and underwater bubble sources may be mechanically tuned over a large frequency band. In one system, a projector changes its resonance frequency by mechanically changing a length of an air-duct between two inside resonators. This tunable bubble seismic source is functional, but turbulent losses require large dimensions for the tunable air duct and for the whole resonator. Furthermore, tunable resonance systems (e.g., high-Q tunable systems) may have many other disadvantages. For example: they may be too sensitive to towing depth and water flow fluctuations; they may have limitations on their frequency sweep rate; they may transmit only specific waveforms with a slowly changing frequency; they may need a special resonant frequency control system to keep the resonant frequency equal to the instantaneous frequency of a transmitted signal; and they may have a large start/stop transient time.
U.S. Pat. Nos. 10,139,503, 9,383,463, respectively, describe a broadband dual bubble resonant system and a bubble resonator excited by an internal motor driven pistons or by valve controlled air flow from a blower. Both systems may be relatively inefficient and expensive; they may use a very complicated nonlinear motor control, either by controlling the motor itself or valves for the blower system. However, such systems may make it difficult to achieve high quality linear sound reproduction. An additional problem may be the noise caused by motor itself. Without special silencers, shock-mounts, and sound isolation from water, the motor noise radiates directly into the water and may cause additional problems for sound receivers with high sensitivity.
Therefore, it appears that an improved underwater sound source having low frequency transmission capabilities would be desirable for marine seismic surveying.